CMOS imagers are increasingly being used as low cost imaging devices. A CMOS image sensor circuit includes a focal plane array of pixel cells, each one of the cells including a photogate, photoconductor, or photodiode overlying a charge accumulation region within a substrate for accumulating photo-generated charge. Each pixel cell may include a transistor for transferring charge from the charge accumulation region to a sensing node, and a transistor for resetting a sensing node to a predetermined charge level prior to charge transference. The pixel cell may also include a source follower transistor for receiving and amplifying charge from the sensing node and an access transistor for controlling the readout of the cell contents from the source follower transistor.
In a CMOS image sensor, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the sensing node accompanied by charge amplification; (4) resetting the sensing node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge from the sensing node. Photo-generated charge may be amplified when it moves from the initial charge accumulation region to the sensing node. The charge at the sensing node is typically converted to a pixel output voltage by a source follower output transistor.
A schematic top view of a portion of a semiconductor wafer fragment containing one exemplary CMOS pixel cell is shown in FIG. 1. The CMOS pixel cell 10 is a four transistor (4T) cell. The CMOS pixel cell 10 generally comprises a charge collection region 21 for collecting charges generated by light incident on the pixel, and a transfer gate 50 for transferring photoelectric charges from the collection region 21 to a sensing node, typically a floating diffusion region 25. The floating diffusion region 25 is electrically connected to the gate of an output source follower transistor 60. The pixel cell 10 also includes a reset transistor 40 for resetting the floating diffusion region 25 to a predetermined voltage before sensing a signal; a source follower transistor 60 which receives at its gate an electrical signal from the floating diffusion region 25; and a row select transistor 80 for outputting a signal from the source follower transistor 60 to an output terminal in response to an address signal.
FIG. 2 is a diagrammatic side sectional view of the pixel cell 10 of FIG. 1 taken along line A-A′. As shown in FIG. 2, the exemplary CMOS pixel cell 10 has a pinned photodiode (PPD) as the charge collection region 21. The PPD 21 is termed such since the potential in the photodiode is pinned to a constant value when the photodiode is fully depleted. The PPD 21 is adjacent to the gate 50 of a transfer transistor. The PPD 21 has a photosensitive or p-n-p junction region comprising a p-type surface layer 24 and an n-type photodiode region 26 within a p-type active layer 20.
Generally, in CMOS image sensors such as the CMOS image sensor cell 10 of FIGS. 1-2, incident light causes electrons to collect in region 26. A maximum output signal, which is produced by the source follower transistor having gate 60, is proportional to the number of electrons to be extracted from the region 26. The maximum output signal increases with increased electron capacitance or acceptability of the region 26 to acquire electrons. The electron capacity of pinned photodiodes typically depends, among other factors, on the doping level of the image sensor and the dopants implanted into the active layer.
Typically, the p-type layer 24, the n-type region 26 and the floating diffusion region 25 are formed within a doped well, for example a p-type well for NMOS transistors or an n-type well for PMOS transistors, located within substrate 20. This well is typically formed prior to the formation of the transfer gate 50 by implanting dopants of a predefined conductivity type within the substrate 20. As known in the art, both the location and dopant concentration of the well affect critical parameters of CMOS imagers, such as dark current, lag and quantum efficiency. For example, if the p-type layer 24, the n-type region 26 and the floating diffusion region 25 are all formed within the doped well, then the quantum efficiency of the imager decreases. If, however, the doped well is masked only in the photodiode region so that the doped well extends below the transfer gate 50 and the floating diffusion region 25, then the quantum efficiency is not affected, but the barrier to charge transfer between the photodiode and the transfer gate is undesirably increased. Alternatively, if the doped well is masked totally out of both the transfer gate and the photodiode region, then the transfer gate leakage undesirably increases.
Accordingly, there is needed an improved active pixel photosensor for use in a CMOS imager that exhibits increased barrier control, improved quantum efficiency and improved leakage. A method of fabricating an active pixel photosensor exhibiting these improvements is also needed, as well as a method of forming a doped well with optimized barrier control.